A mounted substrate including a printed board and an electronic components mounted thereon is used in home appliances such as washing machines, refrigerators and air conditioners, and electronic equipments such as televisions, videos, radios, computers, copying machines and communication equipments. In addition to a single layer substrate, a laminate substrate including a plurality of substrates is used in a mounted substrate in order to obtain satisfactory functions. As a method for the conduction between substrates and a method for mounting electronic components on a substrate, examples thereof include a method for connecting by surface mounting and a method for mounting by inserting a terminal in a though-hole of a substrate. As the step of mounting on a printed board, examples thereof include flow soldering, reflow soldering, manual soldering and the like. Of those, the method for mounting by inserting a terminal in a though-hole is used as the mounting of electronic components having a certain degree of a size from the standpoints of connecting strength and the like. Flow soldering is generally used as the mounting step.
For example, Patent Document 1 discloses Sn—Cu—P—Bi solder alloy as a solder alloy for flow soldering. This solder alloy is excellent in that wettability is improved by the addition of P and additionally a melting temperature is lowered by the addition of Bi, thereby the occurrence of a trouble in operation of electronic components mounted can be suppressed. However, in the case of inserting a terminal in a through-hole and conducting flow soldering, further investigations are required in addition to the improvement in wettability of a solder.
A terminal inserted in a through-hole is connected to a printed board through solder fillet (hereinafter simply referred to as “fillet”) formed between a land of a printed board and the terminal. In such a case, liftoff phenomenon sometimes occurs in the fillet depending on the composition of a solder alloy for forming the fillet and a lamination state of the board. The liftoff phenomenon is a phenomenon that a gap is generated between the land and the fillet, and this phenomenon leads to poor conduction between the board and the terminal. In view of this, various investigations have been made to suppress the liftoff phenomenon.
For example, Patent Document 2 discloses Sn—3Ag-xBi—0.5Cu (x=0, 1, 2, 3 or 4, the unit is mass %) solder alloy in order to maintain high reliability of connection strength of surface-mounted electronic components. This patent document describes the investigation results of the relationship between Bi content and liftoff phenomenon, strength or shrinkage cavity, and discloses the results that those properties are satisfied when the Bi content is 0 mass % or 1 mass %. This patent document further discloses a solidus temperature and a liquidus temperature of those alloy compositions. Patent Document 3 proposes Sn—0.7Cu-(0.35 or 0.7)Bi solder alloy in order to enhance bonding strength without using Ag. This patent document discloses that the occurrence of liftoff phenomenon and the like is accelerated by the addition of Bi. Patent Document 4 proposes that, in Sn—Ag—Bi—In solder alloy, Bi content is from 0.1 to 5 wt % and In content is from 3 to 9 wt %, and additionally a coefficient of linear expansion of a board is a predetermined range, in order to avoid the generation of cracks even after heat cycle in surface-mounted electronic components. This patent document further describes that liftoff phenomenon occurs by the addition of a large amount of Bi.